USB storage count

Here's another USB storage patch. Currently U-Boot handles storage
devices #0 - #4 as valid devices, even if there is none connected. This
patch fixes usb_stor_get_dev() to check detected device count instead
of MAX-define.

This is very important for ill behaving devices. usb_dev_desc[] can be
partially initialized if device probe fails.

After fixing get_dev() it was easy to fix "usb part" etc commands.
Previously it outputed "Unknown partition table" five times, now it's
"no USB devices available".

Signed-off-by: Kim B. Heino <Kim.Heino@bluegiga.com>
diff --git a/common/usb_storage.c b/common/usb_storage.c
index a8642c9..239bddc 100644
--- a/common/usb_storage.c
+++ b/common/usb_storage.c
@@ -175,7 +175,7 @@
 
 block_dev_desc_t *usb_stor_get_dev(int index)
 {
-	return (index < USB_MAX_STOR_DEV) ? &usb_dev_desc[index] : NULL;
+	return (index < usb_max_devs) ? &usb_dev_desc[index] : NULL;
 }